Add a waf --doxygen-no-build option, for a quicker testing loop of doxygen documentation

wscript

@@ -141,6 +141,11 @@ def set_options(opt):
                    help=('Run doxygen to generate html documentation from source comments'),
                    action="store_true", default=False,
                    dest='doxygen')
+    opt.add_option('--doxygen-no-build',
+                   help=('Run doxygen to generate html documentation from source comments, '
+                         'but do not wait for ns-3 to finish the full build.'),
+                   action="store_true", default=False,
+                   dest='doxygen_no_build')
 
     opt.add_option('--run',
                    help=('Run a locally built program; argument can be a program name,'
@@ -560,6 +565,9 @@ def build(bld):
             Options.options.compile_targets += ',ns3module'
         _run_check(bld)
 
+    if Options.options.doxygen_no_build:
+        doxygen()
+        raise SystemExit(0)
 
 def shutdown(ctx):
     bld = wutils.bld